Solid state imaging arrays (ccds, CIDS, etc) have now become the sensors of choice in many applications.
Some CCD devices have been successfully fabricated with over 10.sup.6 operating pixels, each having dimensions &lt;10 .mu.m square. A good example is the Kodak Megapixel device, having 1035.times.1320 pixels of 6.8 .mu.m square in a 7.times.9 mm array. Being planar, geometrically accurate (to the limit of microlithography technology) and with a high quantum efficiency in the visible and near-infrared spectral domains, such devices have the potential to be virtually perfect image detectors.
For the purpose of low-light-level imaging or astrography with CCD devices, the instrument designer's problem is to find an optical system with a matching performance, not only in exceptional resolution and distortion characteristics, but also in speed so as to achieve the highest possible information acquisition rate. When aperture diameters exceed 150 mm, the homogeneity of optical glass becomes an intrusive problem and design solutions usually reduce to catoptric or catadioptric systems which generally require only one refractive component of the full aperture diameter.
Few such systems exist which combine the characteristics of high speed (eg. faster than f/4) and high--and uniform--resolution to the dimensional limit required by CCD pixel structures. If, to these notional constraints, there are added such pragmatic aspects as ease of fabrication and moderate tolerances, the list of suitable designs tends toward zero length.
The top of the list is occupied by the Schmidt camera and its variations; however, as design parameters tend towards higher speed and uniform flattened field resolution, the limitations of the full-aperture aspheric corrector become evident in the form of more difficult and expensive fabrication, significant residual sphero-chromatic aberration and obliquity effects.
Maksutov camera designs also suffer from problems associated with their massive full-aperture thick meniscus corrector component; to such an extent that the advantage of smaller obliquity effects is overridden by sphero-chromatism as the design speed is increased.
An additional obstacle which some low-light-level designs must surmount is the need to accommodate a cryostat for the CCD. Ideally this requires that the focus be accessible externally, which in turn implies a Cassegrain system, or at least a folded format.
This invention is an optical design which is novel in its assembly of known techniques into a format that fits a previously unoccupied area of the speed/diameter relationship. Furthermore, it is a high speed, optical system of economic construction.